The AC output signal from an electronic device which responds to an AC input signal is affected by the load on the output signal, the design constraints on the electronic device, and extraneous degradation such as noise or parasitic capacitances which may be picked up by or inherent in the apparatus which carries the output signal. Each of these factors degrades the amplitude and phase accuracy of the output signal relative to that desired from the input signal.
The accuracy with which the output signal corresponds to the input signal, however, is often important to the use of the electronic device. For this reason, for example, the distortion of the output signal relative to the input signal is a well-known way of gauging the quality of amplifiers.
One common use of amplifiers in which the accuracy of the output signal relative to the input signal is important is in high fidelity sound playback equipment. In these popular components, recorded or broadcast music (and voice) signals are amplified to drive a loudspeaker. The amplifier must accurately amplify the input signal so that the loudspeaker can accurately reproduce the recorded sound.
Design constraints of both cost and technical considerations such as gain, however, often require a trade or compromise between the cost of the amplifier or its gain and the accuracy of signal reproduction. Simiarly, design considerations of cost or space often make it difficult to fully shield the output side of the amplifier from noise and parasitic capacitances which degrade the output signal. These factors, however, arise substantially within the amplifier component and thus, within the design constraints, can be controlled by the manufacturer.
Many high fidelity amplifier components are sold separately from the loudspeaker loads to be driven by the amplifier. Inasmuch as all loudspeakers are not alike, the exact characteristics of the load to be coupled to the component cannot be determined in advance. In addition, the load characteristics of loudspeakers are not uniform (as is often assumed in the design of high fidelity amplifiers). Many speakers, for example, have a resonant frequency within the range of audio frequencies which they reproduce which significantly affects the electrical characteristics of the speaker load. In addition, many loudspeakers have frequency responsive crossover networks between an array of individual speakers within the loudspeaker which can further change the electrical characteristics of the loudspeaker load in response to the frequency of the output signal.